Personally I'd consider him the second (after Thors) or the third (after Bjorn perhaps) introduced with OP capabilities, as Thors pretty much took on everyone 1v1 and was still standing for a while after a few dozen arrows like they were mosquito bites. Bjorn with those berserker mushrooms seemed to be a force to reckon with, except he went against OP Thors so I can't really tell just how OP he is.
But yeah Thorkell is a damn monster. Two fingers cut off and he's having fun. Considering how everyone reacts to the name of Thors though (including Thorkell), I'd make a wild guess that Thors could beat him, Bjorn not so much if they didn't throw him in there to take him out.